Recent observations of ${}^6\mathrm{Li}$ in metal poor stars suggest a large production of this isotope during big bang nucleosynthesis (BBN). In standard BBN calculations, the ${}^2\mathrm{H}(\alpha ,\gamma ){}^6\mathrm{Li}$ reaction dominates ${}^6\mathrm{Li}$ production. This reaction has never been measured inside the BBN energy region because its cross section drops exponentially at low energy and because the electric dipole transition is strongly suppressed for the isoscalar particles ${}^2\mathrm{H}$ and $\alpha$ at energies below the Coulomb barrier. Indirect measurements using the Coulomb dissociation of ${}^6\mathrm{Li}$ only give upper limits owing to the dominance of nuclear breakup processes. Here, we report on the results of the first measurement of the ${}^2\mathrm{H}(\alpha ,\gamma ){}^6\mathrm{Li}$ cross section at big bang energies. The experiment was performed deep underground at the LUNA 400 kV accelerator in Gran Sasso, Italy. The primordial ${}^6\mathrm{Li}/{}^7\mathrm{Li}$ isotopic abundance ratio has been determined to be $(1.5\pm 0.3)\times {10}^{-5}$, from our experimental data and standard BBN theory. The much higher ${}^6\mathrm{Li}/{}^7\mathrm{Li}$ values reported for halo stars will likely require a nonstandard physics explanation, as discussed in the literature.

The rate of the hydrogen-burning carbon-nitrogen-oxygen (CNO) cycle is controlled by the slowest process, ${}^{14}\mathrm{N}(p,\gamma ){}^{15}\mathrm{O}$, which proceeds by capture to the ground and several excited states in ${}^{15}\mathrm{O}$. Previous extrapolations for the ground state contribution disagreed by a factor 2, corresponding to 15% uncertainty in the total astrophysical $S$ factor. At the Laboratory for Underground Nuclear Astrophysics (LUNA) 400 kV accelerator placed deep underground in the Gran Sasso facility in Italy, a new experiment on ground state capture has been carried out at 317.8, 334.4, and 353.3 keV center-of-mass energy. Systematic corrections have been reduced considerably with respect to previous studies by using a Clover detector and by adopting a relative analysis. The previous discrepancy has been resolved, and ground state capture no longer dominates the uncertainty of the total $S$ factor.

The ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction plays a key role in various astrophysical scenarios, from asymptotic giant branch stars to classical novae. It affects the synthesis of rare isotopes such as ${}^{17}\mathrm{O}$ and ${}^{18}\mathrm{F}$, which can provide constraints on astrophysical models. A new direct determination of the $E_R=64.5\mathrm{keV}$ resonance strength performed at the Laboratory for Underground Nuclear Astrophysics (LUNA) accelerator has led to the most accurate value to date $\omega \gamma =10.0\pm 1.4_{\mathrm{stat}}\pm 0.7_{\mathrm{syst}}\mathrm{neV}$, thanks to a significant background reduction underground and generally improved experimental conditions. The (bare) proton partial width of the corresponding state at $E_x=5672\mathrm{keV}$ in ${}^{18}\mathrm{F}$ is ${\mathrm{\Gamma }}_p=35\pm 5_{\mathrm{stat}}\pm 3_{\mathrm{syst}}\mathrm{neV}$. This width is about a factor of 2 higher than previously estimated, thus leading to a factor of 2 increase in the ${}^{17}\mathrm{O}(p,\alpha ){}^{14}\mathrm{N}$ reaction rate at astrophysical temperatures relevant to shell hydrogen burning in red giant and asymptotic giant branch stars. The new rate implies lower ${}^{17}\mathrm{O}/{}^{16}\mathrm{O}$ ratios, with important implications on the interpretation of astrophysical observables from these stars.